74 DYNAMICAL GEOLOGY. 



and coming up to the surface, and the pressure being 

 relieved, a portion of the C0 2 will escape, with efferves- 

 cence. Thus are formed carbonated springs, often called 

 soda springs, on account of their pleasant pungency, like 

 the so-called soda water of the shops. Now, if such water, 

 thus charged with C0 2 , meets limestone, it will dissolve 

 a proportionate amount of this substance. On coming 

 to the surface, the pressure being relieved and the C0 2 

 escaping, the lime carbonate will be deposited about the 

 spring and in the course of the issuing stream as long as 

 the C0 3 continues to escape. 



In this way immense deposits, several hundred feet 

 thick and many miles in extent, are formed. If the 

 deposits are regular and slow, the rock formed will be 

 hard (travertine), but, if rapid and with escaping gas, it 

 is spongy (calcareous tufa). If the water be free from 

 coloring-matter, the stone is exquisitely white and fine ; 

 but if iron be present, it will be yellowish, buff, or 

 brown. If the coloring-matter varies from time to time, 

 the most exquisitely banded, striped, and clouded appear- 

 ance results. Nothing can exceed the delicate beauty of 

 these deposits in some cases. If the water, thus highly 

 charged with lime carbonate, makes a cascade, every 

 object on which the spray falls becomes covered with 

 deposit. In Italy, advantage is taken of this property to 

 make facsimiles of coins, medallions, etc. The stream 

 from the spring is made to fall on lattice, which scatters 

 the spray in all directions. The medallions are placed 

 on shelves within reach of the spray, and quickly be- 

 come incrusted. The removed crust, similarly placed, 

 is used as a mold, in which, by deposit, a facsimile is 

 made. 



Examples of such deposits are found in all parts of the 

 world. Perhaps the most beautiful occur in Italy, where 

 exquisite works of art are made of them. But many 

 examples are found in our own country. About the Old 



